1. Field of the Invention
The present invention relates to a method and an apparatus for drying a coating film, particularly to a method and an apparatus for drying the surface of a long and wide coating film formed by applying various liquid compositions to a continuously running band-shaped flexible substrate (hereinafter referred to as “web”) and a method for producing an optical film.
2. Description of the Related Art
As a method and an apparatus for drying the surface of a long and wide coating film formed by applying various liquid compositions to a continuously running web, there is known a drying method in which a non-coated surface of the web is supported by a roll and the web is dried by blowing air onto a coated surface from an air nozzle; or a non-contact air floating drying method in which a web is dried by blowing air onto both a coated surface and a non-coated surface of the web from air nozzles in a state where the web is floating in the air (Japanese Patent Publication No. 48-042903).
In these methods of blowing air for drying (hereinafter referred to as “hot-air drying method”), drying is generally performed by blowing humidity-controlled air onto a coated surface to evaporate a solvent contained in the coated surface. Although these hot-air drying methods are superior in drying efficiency, they had a problem of failing to provide a uniform coating layer, because air is caused to blow onto the coated surface directly or via a porous plate or a straightening plate and thereby the coated surface is disturbed to make the thickness of the coating layer nonuniform to cause unevenness, and besides, the evaporation rate of the solvent at the coated surface is made uneven by convection of air to cause so-called orange peel defects (refer to Yuji Ozaki, “Coating Kogaku (Coating Engineering)”, Asakura Shoten, 1971, p. 293-294) or the like.
The generation of such unevenness is remarkable especially when the coating solution contains an organic solvent. This is because, when the coating film at the initial stage of drying, which contains sufficient organic solvent, has the distribution of evaporation from the organic solvent, the surface of the coating film comes to have a temperature distribution and a surface tension distribution; as a result, there occurs an in-plane flow, such as the so-called Marangoni convection, in the coating film. This unevenness results in serious coating defects. When a liquid crystal is contained in the coating film, there was a problem such as generation of deviation in the alignment of the liquid crystal in the coating film surface by the blown air in addition to the problem of drying unevenness as described above.
As a method for solving these problems, Japanese Patent Application Laid-Open No. 2001-170547 (pages 3 to 5, FIG. 1) discloses a system in which a dryer is provided immediately after coating. Therein is disclosed a method for preventing generation of unevenness by partitioning the dryer into several parts and carrying out drying in each of the partitioned parts by blowing air from one edge to the other edge in the width direction of a substrate while controlling air velocity. For the same purpose, Japanese Patent Application Laid-Open No. 9-73016 (page 5, FIG. 5) discloses a method of placing metal gauze instead of partitioning the dryer for the same purpose.
Further, there is known a method of increasing the viscosity of a coating solution by increasing the concentration of the coating solution or by adding a thickener to the coating solution to thereby suppress a flow of the coating film surface immediately after coating by drying air, and a method of using a high boiling solution to thereby prevent generation of unevenness through leveling effect of the high boiling solution even if drying air causes flow in the coated surface immediately after coating. However, the method of increasing the viscosity of a coating solution or using a high boiling solution, as described in Japanese Patent Application Laid-Open No. 2001-170547 (pages 3 to 5, FIG. 1), had problems of bringing about a loss of suitability for high-speed coating, an increase in drying time and an extreme drop in production efficiency.
Japanese Patent Application Laid-Open No. 2000-157923 (pages 2 and 3, FIG. 1) discloses a method of controlling air velocity immediately after coating to a small value in order to prevent nonuniform drying of a coated surface by drying air. Further, GB Patent No. 1401041, U.S. Pat. Nos. 5,168,639, and 5,694,701 each disclose a method of drying a coating film without blowing air. Specifically, GB Patent No. 1401041 discloses a method of drying by evaporating a solvent in a coating solution without blowing air and recovering the solvent evaporated. According to this method, an inlet and exit for the passage of a substrate into and out of a casing are provided at the upper portion of the casing; a coating film on the substrate is dried by heating the non-coated surface of the substrate in the casing to promote the evaporation of the solvent from the coated surface; and the solvent evaporated undergoes condensation on a condenser plate disposed at the coated-surface side and is recovered in a condensed state. Further, U.S. Pat. No. 5,168,639 discloses a method of recovering a solvent by using a drum disposed above the upper side of a substrate running in a horizontal direction. Furthermore, U.S. Pat. No. 5,694,701 suggests how to improve the layout disclosed in U.S. Pat. No. 5,168,639.
However, in the method described in GB Patent No. 1401041, since a high-temperature material such as hot water is used for heating and the material is used in contact with or in the extreme vicinity of a film to be dried, the surface temperature of the film during drying rises to a significantly high temperature. This is good in terms of promotion of drying. However, in practice, when the surface temperature of the film rises to too high, a solvent from the coating film evaporates at a very high rate to facilitate generation of nonuniformity in drying, or viscosity of the coating film is reduced with the increase of temperature to thereby cause a flow in the coating film to cause unevenness. On the other hand, if a heating device is not used, the temperature of a coating film is reduced due to the evaporation of a solvent. This caused problems such as a significant reduction in drying rate in the later half of the dryer, generation of blushing phenomena and the like.
As a method for solving these problems, various methods have been proposed in which the non-coated surface of a web is heated by an infrared heater (refer to Japanese Patent Application Laid-Open Nos. 2004-290776, 2003-93953, 5-8372, and 11-254642).
For example, Japanese Patent Application Laid-Open No. 2004-290776 describes a drying method in which drying is performed by providing a dryer, at a running position immediately after coating, which is surrounded by a casing and is provided with an infrared heater for drying, and a hot-air drying device downstream of the dryer. This drying method allows efficient drying of a coating film on a band-shaped flexible substrate without causing drying unevenness in the coating film by heating it so that the difference between the coating film temperature T1 at the inlet of the dryer and the coating film temperature T2 at the outlet of the dryer comes to 5° C. or less.